Gate follow-up to #7649. _cronScheduleForEdit() consulted sched.expr before
schedule_display, so opening or duplicating a job scheduled as
`every monday 9am` populated the field with the canonical `0 9 * * 1`. The
Agent rebuilds schedule_display from whatever the WebUI submits, so the
rewrite stuck and the user silently lost their phrasing.
Only the `once at ...` label is unparseable; every other schedule_display
form round-trips through parse_schedule(), so prefer it and fall back to
expr/expression.
The existing tests asserted on the SOURCE TEXT of the helper and stayed green
through this defect. Adds four tests that EXECUTE the real helper out of
panels.js via node; the natural-language case is red before this fix.
Co-authored-by: happy5318 <happy5318@users.noreply.github.com>
